Some substances emit electrons when exposed to heating or to irradiation of light after first being irradiated by X-rays or other radiation rays (referred to simply as X-rays hereinafter). The emitted electrons are called as exoelectrons.
This invention concerns a novel method of forming X-ray images making use of this phenomenon. Methods of forming X-ray images known so far include exposure of a conventional silver halide emulsion film to X-rays to obtain images containing silver particles in the silver halide emulsion, a xeroradiographic method utilizing the X-ray sensitivity of photoconductors, an ionographic method utilizing the ionization phenomenon of gas or insulating liquid confined between electrodes opposing each other and the like. The method of using silver halide emulsion film has, however, a disadvantage that the process is laborious and takes much processing time, as well as results in waste liquor containing noxious substances because chemical treatments are necessary in each of the stages of development and fixing. Moreover, this method is not very suitable to form X-ray images from soft tissue parts of a body such as muscles, stomach, intestines and the like although suited to form images from skeletal regions or hard tissue parts such as the head, chest, hands and arms, feet and the like.
In xeroradiography or ionography wherein the X-ray images for the object to be inspected are obtained as electrostatic latent images in proportion to the amount of the X-rays transmitted through the object, satisfactory images are obtainable being highly descriminative of the soft parts where the contrast is relatively low due to the edge effect well known in the art of the electrophotography. The xeroradiography however has a defect that sensitivity is not good because the photon efficiency of photoconductors is generally low thereby presenting problems such as excess radiation dose to objects. In ionography wherein ion pairs are produced from gas or insulating liquid confined in the gap between opposing electrodes and drifted to the electrode at the absorption of X-rays, resolving-power is decreased by the scattering of secondary electrons, or the apparatus becomes voluminous and complicated because of the requirements for keeping air tightness in a gas ionographic device or stable insulation in a liquid ionographic device.